On account of the increases in the air freight market, a trebling of the volume of freight is expected over the next 20 years, which in turn will create a demand for approximately 3000 further freight aircraft. Since only few freight aircraft companies buy new freight aircraft from the manufacturers, by far the majority of all freight aircraft are converted passenger aircraft. During the conversion, the core is taken out of the passenger aircraft and all fixtures and fittings necessary for passenger service, such as the galley and panellings, are removed. A large cutout for the freight door is made in the fuselage, the bottom of the cabin is reinforced and the windows are closed by metal covers.
During the conversion to the freight version, the first step taken in the area of the windows of the passenger aircraft is to remove the window retainers and then take out the transparent interior and exterior window panes together with the window seal, known as the window assembly. The actual window frame, which serves for reinforcing the fuselage structure in the area of the window frame and takes up all the structural loads, remains in place.
The replacement of the windows by metal covers means additional weight, causing losses in payload of several tonnes in comparison with freight aircraft that are already supplied ex works without windows. This results in considerable extra costs over the remaining lifetime of the converted aircraft, which can be at least partly compensated by means of the window replacement according to the invention.